Technical Abstract:
Biological control of soilborne pathogens by 2,4-diacetylphloroglucinol (DAPG)-producing isolates of Pseudomonas fluorescens (rhizobacteria) offers a sustainable means of controlling root diseases, but efficacy of the isolates is subject to environmental factors. Our laboratory is identifying host traits that govern interactions with rhizobacteria, with the view to improving biological control in the field. We measured populations of two isolates of P. fluorescens on soil-grown roots of 28 cultivars of Triticum aestivum L. (hexaploid wheat). Isolate Q8r1-96 persisted in the rhizosphere of most cultivars longer than isolate Q2-87, but certain cultivars supported high populations of both isolates, whereas others supported low populations of both. Differential rhizosphere colonization was not due to differences in aggressiveness, as both Q8r1-96 and Q2-87 rapidly colonized wheat roots within four days of seed inoculation. Using three cultivars representing each colonization phenotype, we examined the relationship between cultivar and bacterial isolate in accumulation of DAPG, a pathogen-suppressive polyketide metabolite known to exert phytotoxic effects. Accumulation of DAPG was higher on roots colonized with Q8r1-96, compared to Q2-87, four days after seed inoculation and growth in Petri plates. These results were consistent with the relative production of DAPG by the isolates in culture. However, DAPG production by Q8r1-96 was much greater than Q2-87 on cv. Tara, whereas no difference was observed on cv. Buchanan. The accumulation patterns could not be attributed to differences in rhizoplane population densities, or to the greater root surface area of Tara. Our findings indicate that DAPG accumulation in the wheat rhizosphere is independent of population density, but dependent on an interaction between host and bacterial genotypes. Progress in identifying the host determinants of differential accumulation using micorarrays and other approaches will be presented.